Slidable Fixation Device for  Securing a Medical Implant

ABSTRACT

A fixation device for retaining a leadless medical implant to tissue includes an array of elongate tines having self-expanding distal portions. The fixation tines may be advanced between an implant body and an outer jacket to deploy the tines from a delivery configuration in which the tines are constrained by the outer jacket to an expanded configuration in which the distal end portions of the tines are released from the outer jacket. The implant and fixation device are contained within a sheath for delivery to the treatment site and a pusher within the sheath advances the fixation device relative to the implant body and deploys the tines. A distal end of the implant having an electrode may form a distal tip of the delivery system, and a potential implantation site may be tested prior to deployment of the fixation device to allow for easy repositioning of the implant.

RELATED APPLICATIONS

This application is a continuation-in-part of and claims priority toU.S. patent application Ser. No. 12/759,173, to Erik Griswold, filedApr. 13, 2010.

FIELD OF THE INVENTION

The invention relates to a fixation device for securing a medicalimplant to tissue of the heart.

BACKGROUND OF THE INVENTION

Medical implants such as leadless stimulators or sensors may besurgically, or in some instances, percutaneously delivered and implantedwithin tissue of the heart. The potential for detachment of a leadlessstimulator or sensor from an implantation site represents a serious andpossibly life-threatening event. For example, a leadless pacemaker thatbecomes dislodged from an implantation site in the right ventricle ofthe heart can exit the heart via the pulmonic valve and lodge in thelung. Thus, secure fixation of leadless implants is extremely importantfor successful operation of the implant as well as for the safety of thepatient.

In order to secure the implant to tissue at the implantation site, theimplant may include anchoring structure at a distal end thereof thatmust be screwed or otherwise engaged with tissue at the implantationsite. The anchoring structure is typically housed within a distal end ofa retractable delivery sheath or other covering during delivery of theimplant to avoid injury to the patient as the implant is brought to animplantation site. The anchoring structure is typically deployed tolodge within the tissue by being distally slid and/or rotated relativeto the distal end of the delivery sheath. The delivery sheath is often asomewhat rigid tubular structure and typically includes an open bluntend that may scrape or otherwise cause injury to the patient as thedelivery sheath is being tracked to the implantation site. In the caseof a leadless pacemaker, such a distally placed anchoring structuremakes it difficult or impossible to test the implantation site forresponsiveness to determine whether that area of the heart will acceptpacing pulses until after the full deployment of the anchoring structuresuch that an electrode of the pacemaker makes contact with the heart. Inaddition, if the implantation site is determined to be unacceptable orless than optimal after deployment of the distal anchoring structure, itmay be difficult or impossible to reposition the pacemaker withoutinjury to the heart. Thus a need exists in the art for a delivery andanchoring apparatus and method for delivering and implanting a leadlessimplant in the heart that solves one or more of the deficienciesidentified above.

BRIEF SUMMARY OF THE INVENTION

Embodiments hereof relate to a method of securing a medical implant tobody tissue, particularly to heart tissue. The medical implant may bemounted within an outer sheath of a delivery system to be percutaneouslydelivered to an implantation site. The medical implant includes afixation device slidably mounted thereon. The fixation device includesan annular collar and an array of self-expanding tines that distallyextend from the collar, wherein the tines are constrained andsubstantially straightened by the outer sheath during delivery. When themedical implant is situated at the implantation site, the fixationdevice is distally advanced over the medical implant to distally extendthe tines from the outer sheath into contact with body tissue. Oncereleased from the constraint of the outer sheath, as the tines penetrateor otherwise engage body tissue they tend to splay outwardly from themedical implant and curve backwards toward the collar to retain themedical implant at the implantation site. In an embodiment hereof, adistal end of the implant includes an electrode that protrudes out of adistal end of the outer sheath to form a distal tip of the deliverysystem such that the electrode may be utilized for testing theimplantation site for suitability prior to deploying the fixationdevice.

Embodiments hereof also relate to a system for percutaneously deliveringa medical implant to an implantation site and securing the medicalimplant at the implantation site. The system includes an outer sheathhaving a lumen extending there through and a medical implant held withinthe outer sheath lumen at a distal end thereof. A distal end of themedical implant protrudes out of the outer sheath distal end to form adistal tip of the delivery system. The system also includes a fixationdevice having an annular collar and an array of self-expanding tinesthat distally extend from the collar and are preset to splay outwardlyfrom the medical implant and curve backward toward the collar. Thefixation device is positioned proximal of the distal end of the deliverysystem during delivery and, if applicable, during the testing of theimplantation site. The fixation device is slidably mounted on themedical implant such that the fixation device may be distally advancedover the medical implant to deploy the tines from a deliveryconfiguration, in which the tines are constrained and substantiallystraightened by the outer sheath, to an expanded configuration, in whichthe tines are released from the outer sheath and tend to assume theirpreset shape.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing and other features and advantages of the invention will beapparent from the following description of embodiments hereof asillustrated in the accompanying drawings. The accompanying drawings,which are incorporated herein and form a part of the specification,further serve to explain the principles of the invention and to enable aperson skilled in the pertinent art to make and use the invention. Thedrawings are not to scale.

FIG. 1 is a side view in partial section of an implant delivery system,wherein tines of a slidable fixation device are constrained in adelivery configuration.

FIG. 2 is a side view in partial section of the implant delivery systemof FIG. 1, wherein the tines of the slidable fixation device areextended in a deployed configuration.

FIG. 3 is a perspective illustration of the slidable fixation device ofFIG. 1 removed from the implant delivery system.

FIG. 4 is a side view in partial section of a distal portion of animplant delivery system according to another embodiment hereof, whereintines of a slidable fixation device are constrained in a deliveryconfiguration.

FIG. 5 is a side view in partial section of the distal portion of theimplant delivery system of FIG. 4, wherein only distal portions of thetines of the slidable fixation device are extended in a deployedconfiguration.

FIG. 6 is a side view in partial section of a distal portion of animplant delivery system according to another embodiment hereof in whicha slidable fixation device is retractable, wherein the tines of theslidable fixation device are constrained in a delivery configuration.

FIG. 7 is a side view in partial section of a distal portion of animplant delivery system according to yet another embodiment hereof inwhich a slidable fixation device is retractable, wherein the tines ofthe slidable fixation device are constrained in a deliveryconfiguration.

FIG. 8 is an isometric view of a distal portion of an implant deliverysystem according to another embodiment hereof, wherein tines of aslidable fixation device are constrained in a delivery configuration.

FIG. 9 is an isometric view of the implant of FIG. 8, wherein onlydistal portions of the tines of the slidable fixation device areextended in a deployed configuration.

FIG. 10 is an exploded view of the implant of FIGS. 8 and 9.

DETAILED DESCRIPTION OF THE INVENTION

Specific embodiments of the present invention are now described withreference to the figures, wherein like reference numbers indicateidentical or functionally similar elements. The terms “distal” and“proximal” are used in the following description with respect to aposition or direction relative to the treating clinician. “Distal” or“distally” are a position distant from or in a direction away from theclinician. “Proximal” and “proximally” are a position near or in adirection toward the clinician.

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. Although the description of the invention is in the contextof placement of a leadless pacemaker for treatment of the heart, theinvention may also be adapted for use in delivering and implantingmedical sensors or stimulators to other areas of a patient's body whereit is deemed useful. Furthermore, there is no intention to be bound byany expressed or implied theory presented in the preceding technicalfield, background, brief summary or the following detailed description.

Embodiments hereof relate to systems and methods for delivering andsecuring or anchoring a medical implant within body tissue, such astissue of the heart. FIGS. 1 and 2 are side views in partial section ofan implant delivery system 100 for delivering and implanting implant102. In embodiments described herein, implant 102 is a leadless pacingsystem of the type described in U.S. Pat. No. 5,193,539 to Shulman etal. In the instant disclosure, implant 102 may be delivered andimplanted percutaneously in the heart in a minimally invasive procedurevia delivery system 100. For purposes of describing the inventionhereof, only the basic structure of implant 102 is described herein.Particularly, implant 102 includes at least two electrodes and agenerally cylindrical, capsule-shaped housing 108 that hermeticallyencloses the pacing system's electrical components, including a wirelesscommunication system and an internal power source. A first electrode 120extends distally from a distal end 106 of implant 102 and a secondelectrode 121 having a ring-shape to encircle housing 108 is positionedjust proximal of implant distal end 106. Electrodes 120, 121 areconnected to the electrical components within housing 108 withfeed-through ports (not shown). In an alternative embodiment (notshown), a second electrode may comprise an entire portion of a titaniumhousing that is separated from a distal tip electrode by an insulatingring. When implanted at an implantation site in the heart, first distalelectrode 120 of implant 102 is in intimate contact with the heart wallincluding the myocardium, the thickest and middle layer of the tissue.Implant 102 is sized to be tracked through the vasculature, i.e.,through a femoral vein, a femoral artery, or the subclavian, withindelivery system 100 and may have a diameter or transverse dimension ofup to 9 mm. In accordance with embodiments hereof, implant 102 may bedelivered through the vasculature to be implanted at a septum of theheart or at the apex of the right ventricle. In other embodiments,implant 102 may be implanted within another heart chamber on either sideof the heart. Although implant 102 is described herein as a leadlesspacing system, in other embodiments hereof delivery system 100 may beused to deliver and implant other medical device that are configured tobe secured within body tissue, such as a sensor device or another typeof stimulator device, and may or may not be “leadless” orself-contained.

Implant 102 is retained in heart tissue via a fixation device 110, whichis shown in a delivery configuration in FIG. 1 and an expanded, deployedconfiguration in FIG. 2. With reference to FIG. 3, fixation device 110includes an annular collar 112 for encircling and sliding alonggenerally cylindrical implant housing 108 and an array of prongs ortines 114 that extend from an edge 118 of collar 112. Fixation device110 is formed of a shape memory material and tines 114 are preset tosplay outwardly from implant 102 and to curve backwards toward collar112. In an embodiment, fixation device 110 is formed from aself-expanding material including but not limited to spring temperstainless steel or a stress induced martensite “superelastic” materialsuch as nitinol, a binary alloy of nickel and titanium. “Self-expanding”as used herein means that tines 114 of fixation device 110 have amechanical memory to return to the expanded, deployed configurationshown in FIGS. 2 and 3 from the straightened delivery configurationshown in FIG. 1. Mechanical memory may be imparted to tines 114 offixation device 110 by thermal treatment to achieve to set a shapememory in a susceptible metal alloy, such as nitinol or a spring temperstainless steel. In an alternate embodiment, tines 114 may be separatelyformed of a shape memory material and attached to collar 112 formed of adifferent material via any suitable mechanical method. Although fixationdevice 110 is shown in the embodiment of FIG. 3 with eight tines, itwill be apparent to those of ordinary skill in the art that fixationdevice 110 may include more or fewer tines. As noted above, collar 112is slidingly disposed over housing 108 rather than being longitudinallyfixed relative thereto. In the illustrated embodiment, inner diameter ofcollar 112 is only slightly smaller than an outer diameter of housing108 such that although collar 112 is slidable along housing 108 with theapplication of a pushing force, there is a light interference fitbetween collar 112 and housing 108 for reasons that will be describedbelow.

Implant delivery system 100 includes an outer containment sheath 124having a proximal end 126, a distal end 128, and a lumen 127 extendingtherethrough and an inner pusher 130 having a proximal end 132, a distalend 134, and a lumen 133 extending there through. Pusher 130 isconcentrically disposed within containment sheath 124 and is sized toslidably extend through lumen 127 thereof. Containment sheath 124 andpusher 130 may be formed from tubes or tubing of a flexible polymericmaterial such as polyethylene terephthalate (PET), polyamide,polyethylene, polyethylene block amide copolymer (PEBA), or combinationsthereof. Implant 102 is at least partially contained within a distalportion of containment sheath lumen 127 with a proximal edge 116 offixation device collar 112 positioned toward proximal end 104 of housing108 to be contactable by distal end 134 of pusher 130 and with tines 114of fixation element 110 distally extending in a straightenedconfiguration against implant housing 108. As will be explained in moredetail herein, containment sheath 124 serves to constrain tines 114 offixation device 110 in the delivery configuration of FIG. 1 and pusher130 serves to distally advance or slide fixation device 110 over implant102 and relative to containment sheath 124 in order to release tines 114into the deployed configuration of FIG. 2. In an embodiment, proximalend 104 of housing 108 may be releasably attached to an optional tetheror core wire 138 that extends through pusher lumen 133. Proximal ends126, 132, 140 of containment sheath 124, pusher 130, and tether 138,respectively, each extend proximally outside of the patient's body suchthat they may be manipulated by a clinician and one or more of theproximal ends may include a handle or knob (not shown) in order tofacilitate securing a longitudinal position or sliding movement thereof

When loaded within implant delivery system 100 for delivery, distal end106 of implant 102 distally protrudes or extends from distal end 128 ofcontainment sheath 124 to form a distal tip of implant delivery system100, as illustrated in FIGS. 1, 4, 6 and 7. In the illustrated examples,the exposed portion of implant distal end 106 may have a smooth roundedprofile such that implant delivery system 100 may be safely trackedthrough a patient's vasculature. As noted above electrode 120 extendsdistally from implant distal end 106 such that when implant 102 istracked to and positioned at a potential implantation site, electrode120 may be contacted with the implantation site to test theacceptability thereof prior to deployment of fixation device 110 andimplant 102. More particularly, electrode 120 functions to continuouslymeasure impedance in order to sense electrical contact with heart tissuesuch that once electrode 120 is in electrical contact with heart tissue,a test electrical pulse may be delivered via electrode 120 to test theresponsiveness of the potential implantation site. If the potentialimplantation site accepts or otherwise responds appropriately to thetest electrical pulse, the site is confirmed as the implantation siteand tines 114 of fixation device 110 may be deployed to secure implant102 at the implantation site. If the potential implantation site doesnot accept the test electrical pulse or is otherwise determined not tobe acceptable, the site may be rejected and implant 102 may be moved toanother potential implantation site and the testing procedure repeateduntil an acceptable implantation site is confirmed. Accordingly, implantdelivery system 100 permits potential implantation sites to be testedwithout fixing implant 102 into heart tissue and implant 102 can beeasily repositioned without damaging the body tissue if the sites arenot optimal.

In an alternative embodiment (not shown), when loaded within implantdelivery system 100 for delivery, distal end 106 of implant 102 may berecessed within distal end 128 of containment sheath 124. When implant102 is tracked to and positioned at a potential implantation site,electrode 120 may be projected distally from implant distal end 106 tocontact the implantation site to test the acceptability thereof prior todeployment of fixation device 110 and implant 102. As in the embodimentdescribed above, if the potential implantation site does not accept thetest electrical pulse or is otherwise determined not to be acceptable,the site may be rejected and implant 102 may be withdrawn into thedistal end 128 of containment sheath 124 and moved to another potentialimplantation site. The advancement and testing procedure may be repeateduntil an acceptable implantation site is confirmed.

When an implantation site is confirmed and it is desired to deployimplant 102, distal end 134 of pusher 130 abuts proximal edge 116 offixation device collar 112 to distally advance collar 112 in thedirection of arrow 136 so that fixation device 110 slides along theouter surface of housing 108 and relative to containment sheath 124 tothereby release and deploy tines 114 from distal end 128 of containmentsheath 124. Deployed tines 114 splay outwardly and distally fromcontainment sheath 124 to engage trabeculae and/or penetrate other hearttissue.

In order to prevent fixation element 110 from sliding off housing 108and to transfer some of the pushing force of pusher 130 to housing 108,a stop 122 protrudes from the outer surface of housing 108 to preventfarther distal advancement of collar 112. Collar 112 is prevented fromsliding backward in a proximal direction due to the light interferencefit between collar 112 and housing 108. In an alternate embodiment,housing 108 may include a notch or groove (not shown) thereon and collar112 may include a tab (not shown) that catches within the notch whencollar 112 is advanced distally to stop 122 to prevent collar 112 fromsliding proximally on housing 108. In the alternative embodiment, collar112 may have either a clearance fit or a light interference fit aboutthe body of housing 108. Other variations of stops, detents, tabs, snapfits, notches and grooves may be used to secure collar 112 againsteither distal or proximal movement along housing 108 once implant 102 isin its deployed configuration. With continued force against pusher 130and/or in combination with proximal retraction of containment sheath 124relative thereto, implant 102 is deployed from delivery system 100. Ifpresent, tether 138 may be proximally tugged to assure implant 102 issecured at the implantation site by fixation device 110 prior to fulldeployment of implant 102. Once anchoring is confirmed, tether 138 maybe disengaged from implant 102. For example, tether 138 may be attachedto the proximal end of implant 102 by mating screw threads such thattether 138 may be rotated or turned about its own longitudinal axis inorder to unscrew the threaded distal end and disengage tether 138 fromimplant 102. Implant delivery system 100 including tether 138, pusher130 and containment sheath 124 may then be retracted and removed fromthe patient leaving implant 102 fixed at the implantation site.

As shown in FIG. 2, implant 102 is securely retained at the implantationsite due to tines 114 of fixation device 110 being secured to the hearttissue 150. Further when tines 114 and implant 102 are deployed asdescribed above, electrode 120 is pushed into the heart wall to beembedded therein. Ring electrode 121, serving as the indifferentelectrode, is disposed on implant 102 at a distance from tip electrode120 and may or may not be in contact with heart tissue depending on thedepth to which implant 102 has penetrated tissue 150. Although implant102 is illustrated in FIG. 2 as being deployed generally perpendicularto the heart wall, the disclosure is not so limited, and variations inthe angle of deployment can also affect whether ring electrode 121contacts heart tissue 150, as shown in FIG. 5.

FIGS. 4 and 5 illustrate a slidable fixation device 410 in accordancewith another embodiment hereof with features of the remainder of thedelivery system being the same as the previous embodiment. Fixationdevice 410 includes an annular collar 412 and an array of deployabletines 414 that extend from an edge 413 of annular collar 412. In thedelivery configuration shown in FIG. 4, implant 402 is loaded within adistal portion of containment sheath 124 such that fixation device tines414 are distally straightened by containment sheath 124 to lie againstimplant housing 408 and such that collar 412 is positioned proximal ofhousing proximal end 404. Fixation device 410 is of a shape memorymaterial and tines 414 function similarly to tines 114 described abovewith an exception being that a substantial portion 415 of each tine 414is preshaped to be straight and only a distal portion of each tine 414is preset to radially curve backwards toward collar 412.

In an embodiment, implant 402 may include a proximally extending guidefilament 417 having a stop 419 at the proximal end thereof. Collar 412has an opening large enough to slidably receive guide filament 417, theopening being small enough to prevent stop 419 from entering. Themovement of collar 412 between stop 419 and implant proximal end 404 islimited by the length of filament 417. Similar to electrode 120,electrode 420 may protrude from containment sheath 124 as shown in orderto sense electrical contact with heart tissue and test the suitabilityof a potential implantation site.

After suitability of the implantation site is confirmed, fixation device410 is distally advanced via pusher 130 to lock implant 402 in position.In an embodiment, guide filament 417 may be substantially wire-like toaid in guiding collar 412 and pusher 130 thereover during advancement offixation device 410. In such an embodiment having a rigid filament 417,an inner pusher (not shown) can be slidably disposed within pusher 130to abut and push against stop 419 to assist in advancing implant 402against tissue 150.

In another embodiment, guide filament 417 may be substantiallysuture-like. In such an embodiment, flexible filament 417 may functionas an extension of tether 138, which may be releasably secured to stop419. Thus, pusher 130 may be guided over the combination of tether 138,stop 419 and filament 417. Tether 138 may also perform the functiondescribed above regarding tugging to test the security of fixationdevice 410 in tissue 150 before severing or otherwise disconnectingtether 138 from stop 419, or disconnecting filament 417 from implant402. If flexible filament 417 and stop 419 are left attached to implant402 after implantation thereof, then filament 417 may be loosely coiledbetween collar 412 and implant proximal end 404. Stop 419 may also serveas an attachment point for possible removal of implant 402, should suchbecome necessary.

As shown in a partially deployed configuration in FIG. 5, portion 415 ofeach tine 414 remains straight and positioned against the outer surfaceof implant housing 408 when containment sheath 124 is removed. Duringdeployment, tines 414 slide along generally cylindrical implant housing408 while fixation device 410 is being advanced distally relativethereto by pusher 130. Advancement of fixation device 420 along housing408 may stop when collar 412 abuts the rounded portion of housingproximal end 404 without collar 412 otherwise sliding distally along thegenerally cylindrical body of implant housing 408. Thus, this embodimentavoids the need for a precise fit, either clearance or interferencebetween collar 412 and the generally cylindrical body portion of housing408. Straight tine portions 415 may be preshaped so as to providefrictional engagement therealong with housing 408. Similar to theembodiments above, various stops, detents, tabs, snap fits, notches andgrooves may be used to secure either collar 412 or tines 414 againsteither distal or proximal movement along housing 408 once implant 402 isin its deployed configuration. Once tines 414 have been secured withinheart tissue, pusher 130 may then be held steady relative to implant 402and containment sheath 124 as containment sheath 124 is proximallyretracted to fully deploy implant 402. In this manner, fixation element410, and more particularly the straight portions 415 of tines 414,envelope or encircle substantially the entire medical implant 402 in abasket-like arrangement to securely hold implant 402 against hearttissue when tines 414 are deployed at the implantation site.

According to another embodiment hereof, the slidable fixation device maybe retractable such that the implant can be repositioned even after thetines are deployed. For example, it may be desirable to reposition theimplant after the tines are deployed if the tines do not properly engageor entangle with the heart tissue or if the implantation site issubsequently determined to be less than optimal. FIG. 6 shows anembodiment in which pusher 630 and collar 612 of fixation device 610 arereleasably coupled together using a version of a bayonet mount to enableproximal retraction of fixation device 610. In this embodiment, pusher630 includes a tab 650 that distally extends from a distal end 634 ofpusher 630. Tab 650 includes a radially extending protrusion 652 that isslidingly received in an L-shaped slot 654 formed within collar 612 offixation device 610. The connection between pusher 630 and collar 612transfers an applied force on pusher 630 to collar 612 to slide fixationdevice 610 along housing 608. If the applied force is a pushing force,then collar 612 is distally advanced along the outer surface of theimplant to extend tines 614 out of containment sheath 124 to transformimplant 602 into its deployed configuration. If the applied force is apulling force, then collar 612 is proximally retracted over the outersurface of housing 608 to retract previously deployed tines 614 backinto containment sheath 124 to their straightened deliveryconfiguration. Pusher 630 may be engaged, disengaged or re-engaged withfixation device 610 using twist-and-push or twist-and-pull techniquesknown to those familiar with bayonet mounts.

In another embodiment as shown in FIG. 7, similar to the embodiment ofFIG. 6, a pusher 730 and a collar 712 of a slidable fixation device 710are connected by version of a bayonet mount using a plurality ofL-shaped slots 754 formed within distal end 734 of pusher 730. A collar712 of a slidable fixation device 710 includes a plurality of extensionsor tabs 750 that extend from an edge 716 of collar 712. Each extension750 includes a radially extending protrusion 752 that is slidinglyreceived and catches within a corresponding L-shaped slot 754 in pusher730 as described above with respect to FIG. 6. Protrusions 752releasably couple pusher 730 to fixation device 710 so that pushing orpulling forces applied to pusher 730 are transmitted to fixation device710 to distally extend or proximally retract tines 714, respectively,relative to implant 702.

In another embodiment, FIGS. 8 and 9 are isometric of an implantdelivery system 800 for delivering and implanting implant 802. Implant802 is retained in heart tissue via a fixation device 810, which isshown in a delivery configuration in FIG. 8 and an expanded, deployedconfiguration in FIG. 9. FIG. 10 illustrates implant 802 in an explodedview where generally cylindrical implant body 808 has a plurality oflongitudinal channels 805 formed along the exterior surface of the body.A fixation device 810 has an array of elongate tines 814 having distalend portions preset to splay outwardly from implant body 808 and tocurve proximally backwards as illustrated in FIGS. 9 and 10. Tines 814are joined together at their proximal ends to form a rounded cage-likestructure for abutment with pusher 130 and for engagement with tether838, as illustrated in FIG. 8. An outer jacket 860 is fixedly mountedaround implant body 808 such that tines 814 are slidably interposed inrespective implant body channels 805 between implant body 808 and jacket860. Thus, tines 814 of fixation device 810 may be distally advancedbetween implant body 808 and jacket 860 to deploy the tines from adelivery configuration in which the tines are constrained andsubstantially straightened by the outer jacket 860 to an expandedconfiguration in which the distal end portions of the tines 814 arereleased from outer jacket 860 to radially curve backward.

Fixation device 810 is formed of a shape memory material and tines 814are preset to splay outwardly from implant body 808 and to curveproximally backwards. Fixation device 810 may be made from the samematerials discussed above regarding other fixation device embodimentsherein and may have a shape memory imparted by the same or similarprocesses. As described above with respect to tines 414, a substantialportion of each tine 814 is preshaped to be straight and only a distalportion of each tine 814 is preset to radially curve backwards. Fixationdevice 810 may be formed from individual tines that are attached to eachother by any suitable method at the proximal end of fixation device 810.In an alternate embodiment, all of tines 814 may be cut as a single unitfrom a flat sheet and formed into the shape of fixation device 810.Although fixation device 810 is shown in the embodiment of FIGS. 8-10with four tines, it will be apparent to those of ordinary skill in theart that fixation device 810 may include more or fewer tines.

The delivery system for implant 802 may be similar to the embodimentsdescribed above with respect to implant delivery system 100. Implant 802is at least partially contained within a distal portion of lumen 127 ofcontainment sheath 124 with a proximal end of fixation device 810directed proximally to be contactable by distal end 134 of pusher 130and with tines 814 distally extending in a straightened configurationwithin implant body channels 805. Pusher 130 serves to distally advanceor slide fixation device 810 along implant body 802 and relative tocontainment sheath 124 in order to release tines 814 into the deployedconfiguration of FIG. 9. The proximal end of fixation device 810 may bereleasably attached to an optional tether or core wire 838 that extendsthrough pusher lumen 133.

Implant 802 includes a stop mechanism to limit the distal or proximaltravel of fixation device 810. In an embodiment, a stop extends into atleast one of implant body channels 805. As illustrated in FIG. 10, apin-shaped stop 822 protrudes radially from the bottom of channel 805.At least one of tines 814 has a longitudinal slot 811 for slidablyreceiving stop 822. The longitudinal movement of slotted tine 814 haltswhen either the proximal or distal end of slot 811 contacts stop 822. Inthe expanded configuration, the stop mechanism of implant 802 canprevent fixation device 810 from being pushed farther along implant body808 than intended, thus controlling how far tines 814 can be extendedaway from implant body 808. The stop mechanism of implant 802 may alsoeffect a secure attachment between pusher 130 and implant 802 forretraction, if desired. As pusher 130 is withdrawn, fixation device 810is pulled proximally via tether 838. In addition to the expectedfriction between tines 814 and outer jacket 860 and implant bodychannels 805, a tine 814 can also apply pulling force to implant body808 when the distal end of a slot 811 contacts stop 822. Other stopmechanism configurations are also possible in implant 802, includingarrangements (not shown) wherein a stop protrudes from one or more tinesto slidably engage with respective fixed-length slots associated withimplant body channel 805.

Outer jacket 860 may be formed from a tube or tubing of metal or of aflexible polymeric material as described above regarding containmentsheath 124 and pusher 130. The jacket may be fixedly mounted aroundimplant body 808 by adhesive, a light press fit, shrink fitting or othersuitable methods. Jacket 860 may also have a plurality of notches 862formed at its distal end and being aligned with respective implant bodychannels 805 such that, in the expanded configuration, the distal endportions of tines 814 are released from the outer jacket 860 through thenotches 862. The distal end of implant body 808 may have an enlargeddiameter portion or shoulder such that the abutment of notches 862against the shoulder forms openings closed on all sides and throughwhich tines 814 may exit.

While various embodiments according to the present invention have beendescribed above, it should be understood that they have been presentedby way of illustration and example only, and not limitation. It will beapparent to persons skilled in the relevant art that various changes inform and detail can be made therein without departing from the spiritand scope of the invention. Thus, the breadth and scope of the presentinvention should not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the appendedclaims and their equivalents. It will also be understood that eachfeature of each embodiment discussed herein, and of each reference citedherein, can be used in combination with the features of any otherembodiment. All patents and publications discussed herein areincorporated by reference herein in their entirety.

1. A method of retaining a medical implant to body tissue, the methodcomprising the steps of: receiving a medical implant comprising: agenerally cylindrical implant body having a plurality of longitudinalchannels formed along an exterior surface of the body; an outer jacketmounted about the implant body; and a fixation device having an array ofelongate tines slidably disposed in the respective channels and beingjoined together at a their proximal ends, each of the tines having aself-expanding distal end portion constrained and substantiallystraightened against the implant body by the outer jacket; positioningthe medical implant at an implantation site; and distally advancing thefixation device such that the tines slide between the implant body andthe jacket to release at least the distal end portions of the tines fromthe outer jacket, wherein the distal end portions of the tines releasedfrom the outer jacket splay outwardly from the implant body and radiallycurve backward to engage with body tissue to retain the medical implantat the implantation site.
 2. The method of claim 1, wherein the medicalimplant is received slidably disposed within a lumen of an outer sheath.3. The method of claim 2, wherein the step of distally advancing thefixation device includes distally advancing an inner pusher slidablydisposed within the lumen of the outer sheath to slide the fixationdevice with respect to the implant body.
 4. The method of claim 3,wherein the pusher and the fixation device are releasably coupledtogether to enable proximal refraction of the fixation device.
 5. Themethod of claim 1, wherein the step of distally advancing the fixationdevice includes advancing the tines until a proximal end of alongitudinal slot in at least one of the tines contacts aradially-extending stop protruding from the implant body channel inwhich the at least one tine slides.
 6. The method of claim 2, wherein arounded distal end of the medical implant protrudes out of a distal endof the outer sheath to form a distal tip of the delivery system whilethe delivery system is being tracked to the implantation site.
 7. Themethod of claim 6, wherein the medical implant is a leadless pacemakerwith an electrode extending from the distal end thereof and the step ofpositioning the medical implant includes sensing when the leadlesspacemaker is in electrical contact with the implantation site within theheart by utilizing the electrode.
 8. The method of claim 6, wherein themedical implant is a leadless pacemaker with an electrode extending fromthe distal end thereof and further comprising the step of testing theimplantation site within the heart for suitability by utilizing theelectrode prior to distally advancing the collar of the fixation deviceover the leadless pacemaker.
 9. The method of claim 8, wherein the stepof testing the implantation site for suitability includes delivering anelectrical pulse to the heart via the electrode to test theresponsiveness of the implantation site.
 10. The method of claim 8,further comprising the step of repositioning the leadless pacemaker ifthe implantation site is determined to be unacceptable.
 11. The methodof claim 1, wherein the implant is a leadless pacemaker and theimplantation site is in the heart.
 12. A system for percutaneouslydelivering a medical implant to an implantation site, the systemcomprising: an outer sheath having a lumen extending there through; amedical implant slidably received within a distal portion of the outersheath lumen, the implant comprising: a generally cylindrical implantbody having a plurality of longitudinal channels formed along anexterior surface of the body, and a rounded distal end of the implantbody that may be positioned to protrude from a distal end of the outersheath to form a distal tip of the delivery system; an outer jacketmounted about the implant body; and a fixation device having an array ofelongate tines having self-expanding distal end portions and beingjoined together at their proximal ends, the tines being slidablydisposed in respective implant body channels such that the tines may bedistally advanced between the implant body and the jacket to deploy thetines from a delivery configuration in which the tines are constrainedand substantially straightened by the outer jacket to an expandedconfiguration in which the distal end portions of the tines are releasedfrom the outer jacket to radially curve backward.
 13. The system ofclaim 12, further comprising an inner pusher slidably disposed withinthe outer sheath lumen for distally advancing the tines of the fixationdevice between the implant body and the outer jacket.
 14. The system ofclaim 13, wherein the pusher and the fixation device are releasablycoupled together to enable proximal retraction of the fixation devicerelative to the outer jacket and the body.
 15. The system of claim 12,wherein at least one of the tines has a longitudinal slot slidable alonga stop protruding from the implant body channel in which the at leastone tine slides such that a proximal or distal slot end is engageablewith the stop to limit longitudinal movement of the fixation device. 16.The system of claim 12, wherein the medical implant is a leadlesspacemaker and an electrode is mounted at the distal end of the medicalimplant.
 17. A medical implant fixation system, the system comprising: agenerally cylindrical implant body having a plurality of longitudinalchannels formed along an exterior surface of the body, and a roundeddistal end of the implant body that may be positioned to protrude from adistal end of the outer sheath to form a distal tip of the deliverysystem; an outer jacket mounted about the implant body; and a fixationdevice having an array of elongate tines having self-expanding distalend portions and being joined together at their proximal ends, the tinesbeing slidably disposed in respective implant body channels such thatthe tines may be distally advanced between the implant body and thejacket to deploy the tines from a delivery configuration in which thetines are constrained and substantially straightened by the outer jacketto an expanded configuration in which the distal end portions of thetines are released from the outer jacket to radially curve backward. 18.The system of claim 17, wherein the outer jacket has a plurality ofnotches at a distal end, the notches being aligned with respectiveimplant body channels such that, in the expanded configuration, thedistal end portions of the tines are released from the outer jacket viathe notches.
 19. The system of claim 18, wherein at least one of thetines has a longitudinal slot slidable along a stop protruding from theimplant body channel in which the at least one tine slides such that aproximal or distal slot end is engageable with the stop to limitlongitudinal movement of the fixation device.